The present invention relates generally to machines for shaping single crystal ingots, and, more particularly to a swivel chuck for holding a conical end of such an ingot while the ingot is being rotated and machined for a desired shape.
Semiconductor substrates that are used to fabricate semiconductor integrated circuits are typically manufactured from single crystal ingots, or boule. These ingots are commonly grown by a standard melt crystal growth technique, such as the Czochralski (CZ) method. In the CZ method a cylindrical single crystal is pulled vertically from silicon melt in a heated crucible. The growth is initiated by dipping a small seed crystal in the melt, and after the thermal equilibrium is reached, the crystal is pulled upwards so that it grows with a constant diameter. At the same time, the crystal rod and the crucible are rotated in opposite directions. This process results in a single crystal ingot that is has a generally constant radius, uniform dopant and impurity distribution, low number of defects, and continuous Czochralski growth.
Once the single crystal ingot is formed further processing is necessary to shape the cylindrical structure into a prescribed diameter. This processing typically involves centering the single ingot crystal in a shaping device, such as a lathe, and then grinding the ingot to the prescribed diameter dimension. Once the ingot has been properly ground to the required diameter, the ingot is then sliced perpendicular to the longitudinal axis to obtain generally planar wafers. The resulting wafers are then further processed to form a multitude of individual integrated circuits.
The single crystal ingots resulting from the CZ method have generally conical ends. These ends are centered in the lathe, using a chuck located at each end thereof to grip the generally conical ends so that the ingot can be ground to a prescribed diameter. Current lathes employ chucks defining concave socket surfaces that are positioned at opposite ends of the lathe (referred to in the art as the headstock and tailstock of the lathe) to hold the conical ends of the single crystal ingot.
The crystal growth process results in the generally conical shaped ends having somewhat irregular shaped surfaces that typically exhibit protruding nodes or other surface anomalies. The end portions of the ingots thus contact the interior surfaces of the conical socket portion of the lathe chuck with irregular concentrated pressure points that are related to the geometric characteristics of the conical ends of the ingot. Axial pressure is exerted between the tailstock and the headstock of the lathe in supporting and rotating the single crystal ingot and such pressure brings the surfaces of the conical ends of the ingot against the surfaces of the chucks used to hold such an ingot in a lathe. Pressure concentrations caused by nodes in the conical end portions of the ingot are thus imposed on the surfaces of such chucks. For a more detailed discussion of the axial pressure exerted during the lathe process see U.S. Pat. No. 5,525,092, entitled xe2x80x9cDevice for Preventing Idle Rotation in Cylindrical Ingot Grinderxe2x80x9d, issued Jun. 11, 1996, in the name of inventors Hirano et al.
Most previously known chucks are of unitary construction and have axially oriented fastener holes that intersect the conical interior socket surfaces. These fastener holes are used to receive fasteners, such as bolts, which connect the chucks to the base of the lathe. In application, an ingot held in such a chuck will typically slip until a node on its surface engages a fastener hole and interlocks with it. While this stops the ingot from further slippage, it often results in the ingot being located skewed from the desired concentric axis of the lathe. Further problems are encountered with these chucks during the machining process, as the ingot is prone to slip into different positions due to the movement of the lathe.
Additionally, because of the previously mentioned irregular surfaces of the conical end portions of the ingot, the surfaces of the interior of the conical chucks are susceptible to damage. This surface damage is especially prevalent at the seat of the holes through which the fasteners are inserted to connect the chucks to the headstock/tailstock of the lathe. The deformation of the material surrounding the holes causes these areas to more likely engage nodes on the ends of ingots, thus impeding accurate placement of the ingot in the chuck. Damaged and/or deformed chucks require replacement and thus bring about unwarranted hardware expenditures, which drive up the cost of wafer production. Additionally, the replacement of such chucks is a time consuming endeavor that requires precise centering on the lathe to mitigate unacceptable vibration in the ingot during the grind process.
To alleviate these problems in most lathe assemblies it is necessary to center the conical ends of the ingots and to securely grip them so that the ingots can be rotated during the grinding operation without excessive slippage. This insures that the ingots can be shaped accurately into the required generally cylindrical shape in preparation for being subsequently sliced into wafers. Using the conventional chucks described above it has been difficult to both center an ingot accurately and to hold the ingot securely enough to rotate it in a controlled manner so that it can be shaped as required without hastening damage to the chuck surfaces that contact the ingot.
An improved chuck device has been developed that utilizes multiple pieces to eliminate the likelihood of fastener holes engaging the nodes on the conical end of the ingot and to minimize replacement and downtime losses related to damaged chucks. See for example, U.S. Pat. No. 6,012,976, entitled xe2x80x9cMulti-Piece Lathe Chuck for Silicon Ingots,xe2x80x9d issued Jan. 11, 2000, in the name of inventors Aydelott et. al. The multiple piece chuck has a base portion that receives fasteners for attachment to the headstock or tailstock of the lathe. A receptacle within the base portion holds an annular chuck insert which has a conical socket surface to engage the conical end portion of the ingot. Setscrews are provided within the base portion and serve to retain the chuck insert in the receptacle portion of the base. This configuration allows for the interior surface of the chuck insert to be continuous and, thus devoid of any fastener holes that are susceptible to engaging nodes on the ingot. Additionally, the dual portion aspect of the chuck allows the chuck insert to be readily replaced without the need to replace the entire chuck.
However, recently developed chucks do not account for the high degree of friction that results between the chuck and the single crystal ingot. If the ingot is not perfectly centered during the initial loading of the ingot into the chuck, the friction between the ingot and chuck prevents the ingot from properly sliding into the proper center position. This problem can not be addressed by increasing the clamping force on the crystal, as the delicate nature of the ingot would lead to cracks in the crystals if the force on the crystal were increased.
What is desired is an improved chuck for holding a conical end of an object, such as a single ingot crystal formed of a semiconductor material. The chuck should be capable of properly securing the object in a centered position and to allow for ample transferring of forces to the object to rotate it as it is machined or otherwise shaped as required. The desired chuck should be able to compensate for eccentric loading of the crystal into the chuck. Additionally, the chuck should provide for a continuous mating surface that does not provide for fastener holes that are otherwise engaged by the irregular surfaces of the conical end of the object. As an added benefit, the chuck should be configured so as minimize cost and downtime related to replacing damaged or exhausted chucks.
The present invention provides for an improved chuck for supporting elongated work objects having conical end portions, such as single crystal ingots used to fabricate semiconductor wafers. The chuck is typically used in a lathe for positioning and allowing rotation of the work piece during a grinding procedure that results in optimal work piece diameter.
The chuck comprises a ring-shaped base, or socket, having a cavity defining a central axis therethrough and a series of fastener holes for securing the base to a headstock or tailstock of a lathe. The base receives within the cavity a portion of a chuck insert. The chuck insert defines a work piece support surface that is coaxial with the central axis when the chuck insert is disposed in a nominal position within the cavity of the base. The work piece support surface will typically define a continuous, arcuate, convex surface capable of supporting work pieces having conical end portions of varying diameters. The continuous, uninterrupted support surface allows for the conical shaped end portions to contact the chuck insert without protruding nodes, or other anomalies, in the work piece engaging fastener bores, or other indentations in the support surface. The arcuate, convex nature of the support surface allows for the chuck to receive conical end portions of varying diameters without the need to change the chuck insert to accommodate the variance in shape of the work piece. Additionally, the arcuate surface minimizes the contact area between the work piece and the chuck so that nodules or other anomalies on the surface of the conical end portions are less likely to interfere and cause the work piece to align improperly within the lathe.
The chuck also includes a retainer that is adapted to surround and support at least a portion of the chuck insert. In a typical embodiment of the invention the retainer will comprise a two-piece unit that assembles to form a continuous ring-like structure around the periphery of the chuck insert. The two-piece embodiment allows for the retainer to be easily removed from the chuck, thus allowing for the chuck inserts to be readily replaced if they become damaged or exhausted. Additionally, the retainer allows the chuck insert to pivot from the nominal position to a predetermined angle relative to the central axis when the conical end of the work piece is loaded into the chuck so as to align the work piece within the lathe. The pivoting nature of the chuck of the present invention allows for proper alignment of the work piece in the lathe. The pivoting action allows the chuck to overcome the frictional forces that exist between the work piece and the support surface during the initial work piece loading process.
Additionally, the chuck of the present invention may comprise a seal that is disposed between the underside of a lip in the chuck insert and the retainer. The seal will typically occupy a groove in the lip of the chuck insert and provide a pivot gap for the chuck insert so that it can pivot a predetermined degree in relation to the central axis. The chuck may also comprise insert pins that are disposed within the chuck insert. Typically, two insert pins will be disposed 180 degrees opposite one another on the exterior surface of the chuck insert. The insert pins will then engage insert slots within the retainer. The insert pins allow for the chuck insert to pivot in a back and forth motion while prohibiting the chuck insert from rotating about the central axis.
As such, the present invention provides for an improved chuck that is capable of pivoting to allow for work pieces to be loaded into the chuck and properly aligned within a lathe. Additionally, the chuck of the present invention provides for a continuous support surface that further aids proper alignment by eliminating the possibility of nodes on the surface of the work piece engaging holes and impeding the proper alignment position. The improved chuck also is designed so as to accommodate ease in chuck insert replacement, thereby limiting lathe downtime and making for a more efficient process.